Reconstitution of T cell immunity following bone marrow transplantation is critical for the protection of the host from both infectious pathogens and tumor recurrence but this process is slow and is limited by parameters which are poorly defined. This protracted period of immunodeficiency is associated with high morbidity and mortality. Further characterization of the mechanisms limiting immune reconstitution is restricted by available experimental systems. To address this issue, we have developed a novel model of in vitro T lymphopoiesis that recapitulates the differentiation of functional T cells in a system that is able to mimic the body's thymus. The system is based on our novel, patented, three-dimensional cell growth scaffold, termed the Cytomatrix TM, which is used to mimic the 3-D architecture of the thymus. In this project we propose to use a well characterized, clinically relevant murine model of allogeneic bone marrow transplantation to evaluate the contribution of these cells to immune reconstitution post transplantation. In Phase 1, we plan to optimize the in vitro lymphopoieis system using strain specific murine tissues followed by in vivo administration in the following aims: AIM 1: To generate T cells in the Cytomatrix culture system from hematopoietic stem cells using murine thymic tissue. AIM 2: To study the contribution of T cells produced in the Cytomatrix system in immune reconstitution following allogeneic bone marrow transplantation. AIM 3: To transplant the T cells produced in the Cytomatrix system into mice undergoing allogeneic bone marrow transplantation and to evaluate the frequency and severity of GVHD. In Phase 2, we plan to evaluate the clinical model by generating murine T cells with the system using Murine skin in place of thymus, and to then administer these to mice undergoing allotransplantation as in Phase 1, and determine the status of immune reconstitution and the frequency of GVHD, as well as responses to viral and bacterial infections and graft versus leukemia effects.